1. Field of the Invention
The present invention relates to an access controller for controlling the seek operation of a head relative to a recording medium.
2. Description of the Related Art
Various apparatuses are known which access the data recorded on recording media. One of these is a magnetic disk drive apparatus having an access controller with an access mechanism. A magnetic head in the access mechanism is moved in the radial direction of a magnetic disk, i.e., the recording medium, and positioned at a desired cylinder on the magnetic disk. Generally, a voice-coil motor is incorporated in the access controller of the disk drive apparatus for accessing disks having a great data storage capacity. FIG. 1 is block diagram showing the conventional access controller including such an access mechanism.
As is shown in FIG. 1, the access controller comprises a magnetic head 110. The head 110 reads the servo data from a magnetic disk 100 of a dedicated servo system, and generates a servo signal representing the servo data. The servo signal is amplified by an amplifier 111 and input to a peak hold (P/H) circuit 112. In response to the timing control signal supplied from a main controller 127, the peak hold circuit 112 samples the peaks of the servo signal. The values of the peaks, thus sampled, are supplied to a position signal generator 113. The generator 113 generates a position signal from the peak values of the servo signal. An actual velocity detector 114 differentiates the position signal, thus producing the actual velocity signal S1 shown in FIG. 2.
In the meantime, a cylinder pulse generator 117 generates a cylinder pulse from the position signal output from the position signal generator 113. The cylinder pulse represents the fact that the head 110 has moved across a cylinder. The cylinder pulse is supplied to a counter 118, whereby the count value of the counter 118 is reduced by one. The count value has been set to the counter 118 by the controller 127 before the seek operation, and represents the number of cylinders over which the head 100 must move from the current position to the destination cylinder. In accordance with the count value of the counter 118, target velocity table 119 outputs the digital data showing a target velocity. A D/A converter 120 converts the target velocity data into an analog signal, or the target velocity signal S2 shown in FIG. 2.
The actual velocity signal S1 and the target velocity signal S2 are supplied to a subtracter 123 via amplifiers 115 and 121, respectively. The subtracter 123 subtracts the actual velocity signal S1 from the target velocity signal S2, thereby producing the velocity difference signal S3 illustrated in FIG. 2. The signal S3 is supplied to a driver 125. The driver 125 supplies a VCM current I to a voice-coil motor (VCM) 126 in accordance with the velocity difference signal S3. The motor 126 moves the magnetic head 100 to the destination cylinder at the target velocity.
As shown in FIG. 2D, the seek time which the head 110 requires to move from the current position to the destination cylinder is the sum of the three periods during which the head 110 is accelerated, moved at a constant velocity, and decelerated. During the acceleration period, the maximum VCM current I is supplied to the voice-coil motor 126, so that the actual velocity of the head 110 increases to the target velocity. The lengths of the acceleration periods differ among the access controllers, since access mechanisms in the controllers have different characteristics. As a result, the seek times differ among the access controllers. When the seek time is too short, a seek error will occur; when it is too long, a reduction of the throughput of the magnetic disk drive apparatus will occur.
In order to prevent such a seek error, or such a reduction of throughput, variable resistors 116 and 122 are used to weight the actual velocity signal S1 and the target velocity signal S2, respectively, that is, to change the amplitudes of these signals. The time for moving the head 110 from the current position to the destination cylinder is thereby made to fall within a predetermined range, though the voice-coil motor and the access mechanism have characteristics different from those of their counterparts used in other controllers. However, it requires time and cost to adjust the resistors 116 and 122, thereby to appropriately change the amplitudes of the signals S1 and S2. Further, since the access mechanism has its characteristics changed with time, the seek time for moving the head 100 from one cylinder to the destination cylinder falls outside the predetermined range.